A motor control system that subjects various machines to driving control includes a host controller and a motor control apparatus. The motor control apparatus drives motors of the various machines based on an operation command from the host controller and the positions and the speeds of the motors. As the motor control apparatus, there is a type for obtaining “the positions and the speeds of the motors” using outputs of detectors attached to the motors and a type for estimating “the positions and the speeds of the motors” from electric currents or voltages of the motors without using the detectors. When the motor control apparatus is a servo amplifier, as the servo amplifier, there is also a built-in positioning type.
In the motor control system, for example, when the various machines are machine tools, from the view point of safety, an operator cannot approach a work piece during motor rotation. For example, in a lathe, a safety fence is kept closed. The lathe is operated in a state in which an operator cannot approach the lathe not only when the lathe is in a high-speed range but also when the lathe is in a low-speed range.
However, actually, for example, during replacement of work, to enable the operator to check a work position, it is necessary to rotate the work in a state in which the safety fence is opened. In this case, it is requested to detect, with high reliability, whether the number of revolutions is actually within a safety range.
To meet this request, a dedicated apparatus for monitoring rotating speed is necessary separately from the motor control apparatus. However, an increase in costs is inevitable. To add the apparatus after the system is once assembled, the entire system needs to be reviewed. Therefore, the addition of the apparatus is practically often impossible.
To solve this problem, various measures for securing safety of the motor control apparatus have been proposed (e.g., Patent Literatures 1 and 2).
Patent Literature 1 proposes a method of predicting current motor speed from a command pattern using a control system model, which simulates a transfer function of a control system, and comparing the predicted current motor speed and motor speed detected by a speed detector to thereby determine abnormality.
Patent Literature 2 proposes, as a method of multiply monitoring a motor rotation state using a plurality of kinds of information, a method of monitoring the motor rotation state using information (the position, the speed, and the magnetic pole position of the motor) from a detector and information concerning a motor rotation state estimated from an induced voltage generated in winding of the motor.